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Author Topic: Thordarson Mod Iron Terminals  (Read 4532 times)
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KD1SH
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« on: May 16, 2020, 10:48:26 AM »

   Looking through some of my recent Nearfest acquisitions, and checking out a nice old Thordarson T-21M65 mod transformer.  I'm sure a great many of you have used similar Thordarson iron - what's up with those terminals that look like heavy duty pin jacks?  Looking in old Thordarson catalogs, I can't find any reference to what sort of plugs go in there, specifically what diameter.  I could always experiment by turning down some rod-stock on my lathe until I find a diameter that seems right, but I don't want to risk breaking those female jacks.

   Edit: Apologies; this probably should have gone in Technical.
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w8khk
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« Reply #1 on: May 16, 2020, 11:24:57 AM »

I am using a 500 watt Thordarson CHT number T-11M78 in my P-P 250-TH rig modulated by a pair of 810s.  I had a shortage of those pins too.

I don't know if the terminal boards on your T-21M65 are the same size, but what I found works very well as a substitute is a regular 6-32 machine screw.  They fit snug, but not too tight.  I put a solder lug on the end of the plate wire, and then a 3/4 inch machine screw and nut holding the solder lug, and just plug it into the transformer jack.  I do not recall the transformer number, but my CHT driver transformer uses the same size pins.  Hope this is helpful to you.
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Rick / W8KHK  ex WB2HKX, WB4GNR
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KD1SH
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« Reply #2 on: May 16, 2020, 02:20:05 PM »

   I just tried it, and it works fine - thanks for the idea.  If I feel like getting fancy sometime, I might try turning down some #10 brass machine screws to the same OD as a #6, but leaving a bit of thread on one end for attaching a lug.  Your #6 screw idea works fine, though - why complicate things?


I am using a 500 watt Thordarson CHT number T-11M78 in my P-P 250-TH rig modulated by a pair of 810s.  I had a shortage of those pins too.

I don't know if the terminal boards on your T-21M65 are the same size, but what I found works very well as a substitute is a regular 6-32 machine screw.  They fit snug, but not too tight.  I put a solder lug on the end of the plate wire, and then a 3/4 inch machine screw and nut holding the solder lug, and just plug it into the transformer jack.  I do not recall the transformer number, but my CHT driver transformer uses the same size pins.  Hope this is helpful to you.
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« Reply #3 on: May 16, 2020, 04:23:55 PM »

I am glad it worked out well for you too, William.
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan

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« Reply #4 on: May 21, 2020, 03:02:40 PM »

A pin from a Molex connector may work well too as it's more spring-ish.
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w8khk
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« Reply #5 on: May 21, 2020, 04:01:55 PM »

A pin from a Molex connector may work well too as it's more spring-ish.

Good idea, but probably not necessary.  The pins supplied with the transformer are a hollow cylinder of fixed diameter, so the wire may be soldered inside.  The receptacles in the transformer have the spring-loaded assembly to provide a tight fit and good conductivity.  It is just a matter of using the right diameter pin.  Unfortunately standard banana plugs are too large, and the smaller pins used in some of the older meter leads are too small.

But of course, if you find the spring in the receptacle weak or damaged, a springy pin plug would be the cat's meow.  (I know you just love cats.  Me too!)
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan

My smart?phone voicetext screws up homophones, but they are crystal clear from my 75 meter plate-modulated AM transmitter
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« Reply #6 on: May 21, 2020, 04:41:42 PM »

Question:

Tnx for the 6-32 screw idea.

I have a friend who can give me a T-11M76  (125w Thor mod xfmr)  to build a 150w class C rig.  Maybe 4D32s X 4D32s or whatever.  I am told the T-11M76 sweeps only down to 150 to 200 Hz without doing gapping mods. I saw nothing in the catalog about audio bandwidth.

Has anyone pulled the bells off and seen whether they are sealed in tar/pitch... or the connections will be damaged when doing so?  

Here is the general technique I am basing this gapping on:

"Remove the end bells or frame and the bar that goes across the E laminations will come out.  You will need an inductance meter to check the inductance before you do this.  Typically it will be between 4 to 8 H.  Reduce the gap spacer until you get 16 to 32H.  You basically  want to up it by about 4X.  This gives some headway for mod tube unbalance.  Then get a mod reactor at least equal the resulting inductance.  All these measurements should be on the secondary.  These changes will take a crappy mod transformer down to 30 to 50 CPS or even less!"

I hesitate to pull the T-11M76 apart until I hear about some actual journeys with this xfmr line.

*Update: I am told there are no bells on this xfmr, just sits in a can, which makes mods more difficult.

T
  
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w8khk
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« Reply #7 on: May 21, 2020, 08:52:32 PM »


I hesitate to pull the T-11M76 apart until I hear about some actual journeys with this xfmr line.

*Update: I am told there are no bells on this xfmr, just sits in a can, which makes mods more difficult.


Tom, my restored P-P 250-TH rig uses the big brother mod transformer, the 500 watt T-11M78, of the Thordarson CHT series.  There are no end bells, instead it has formed (pressed) sheet metal covers on each side, with rounded corners.  There is no obvious way to disassemble it.  And at this point, I do not think it wise to attempt that operation.  The homebrew transmitter was purchased used by my grandfather in 1937, and the transformer is original, making it greater than 83 years old.  There is no telling the condition of the wires (insulation) to the terminal strip, but it functions fine now, so best left alone.  

if the '11M76 is available for reasonable cost, it is certainly worth a try to open up and modify yours.  I have not attempted to obtain very low audio response, due to the assumed fragility of the very old mod iron in my rig.  But I think the CHT line is probably over-built compared to advertised specifications.  CHT is Chester H. Thordarson's "signature", indicating top of the line.  I have several other CHT power transformers and filter chokes, and have never had a failure with this line of iron.
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan

My smart?phone voicetext screws up homophones, but they are crystal clear from my 75 meter plate-modulated AM transmitter
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« Reply #8 on: May 21, 2020, 11:39:19 PM »

In case it is of any use to anyone, I have the 75 W version and I discovered that an RCA phono plug tip fits the patchboard holes perfectly.

Dean
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« Reply #9 on: May 22, 2020, 01:20:34 AM »

Not that it helps in this day and age but the original pins were an Amphenol number and were popular on the end of 1940's police radio control cables where the wires plugged into the back of the control head.  The pins had a plastic or bakelite shoulder on them to grab onto.  I think they were discontinued in the early 50's.  After that the police radios had pins the size of octal tube pins, with cardboard tags rather than plastic.

The original Amphenol numbers were 71-1S (3/32"), 71-1M (1/8"), and 71-1L (5/32".) 5 cents each list price in 1944...
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Geoff Fors
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